BIG PHYSICS, BIG QUESTIONS –

Earth’s water must have arrived here earlier than we thought

Earth’s isotopes are most like those found in this kind of meteorite, from the Sahara

Nicolas Dauphas

By Chelsea Whyte

The arrival of water on our planet wasn’t a last-minute job.

Water came to Earth on icy comets after most of the planet and its core were formed, about 4.5 billion years ago, according to a leading theory.

But now an analysis of isotopes from meteorites born earlier, when the solar system was formed, seems to imply that the wet stuff got here much sooner.

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To pin down when meteorites could have delivered Earth’s water, Mario Fisher-Gödde and Thorsten Kleine at the University of Münster, Germany, looked at the Tagish Lake meteorites that fell in British Columbia, Canada, in January 2000. They compared the abundance of ruthenium isotopes in these meteorites with the abundance in Earth’s mantle.

“Meteorites impacted Earth during formation and they can leave signatures,” says Katherine Bermingham, at the University of Maryland. “Ruthenium isotopes are stable. That means they can act as fingerprints.”

If this kind of meteorite brought water to Earth during a late heavy bombardment, then the isotopes inside them should match the isotopes in Earth’s mantle.

“These isotopes were produced in a stellar environment. Their signature can’t be erased by later processes,” says Fisher-Gödde. “That’s why it’s a good tool.”

But Fisher-Gödde and Kleine found that the ruthenium isotopes were distinctly different in meteorites and those found in Earth’s mantle. “We can exclude a late water delivery,” Fisher Gödde says.

Move the clock back

This doesn’t rule out the possibility that meteorites may have brought water to Earth earlier in its formation, during the growth of Earth’s core and before the impact that formed the moon, about 30 to 50 million years after the origin of the solar system.

“The conclusions do align with our research, in that they predict Earth’s water must have been delivered during accretion, rather than later on,” she says. “The ruthenium data suggest comets could not have played a large part in the late addition of material to Earth.”